Genesis and Controls on the Mineralization of the Keglovic Deposit, South-Central Yukon

• Author / Creator

  Pryer, Laurence W

• The mid-Cretaceous Ag-Pb-Zn-Cu-Sn Keglovic deposit, located in the south-central Yukon, is formed by the superposition of an earlier Pb-Zn skarn system with later Keno Hill-style Ag-Pb-Zn veins and is the product of complex regional geological processes, which occur synchronously with mineralization. The Keglovic deposit is located in the Tay River district, which contains a unique assemblage of mid-Cretaceous (120-90 Ma) igneous rocks belonging to the Omineca Magmatic Belt (OMB). The tectonic setting of plutons in the OMB is controversial. All previous attempts to model the OMB assume that the plutons within it form continental margin-parallel bands, which migrate systematically inboard (away from the margin) with time. This study demonstrates that plutonism in the Tay River district spans a period from 109–90 Ma with no inboard migration, violating this assumption. Igneous activity in the Tay River district can be divided into three suites. The oldest plutons are 109–104 Ma S-type granites belonging to the Anvil suite, which derived from sediment melting in an exhuming thickened crust. This study uses a novel method to correct discordant U-Pb analyses from xenocrystic zircons from these S-type granites, allowing the identification of the Hyland Group as the protolith for the Anvil suite magmatism. The second suite are 100–96 Ma Tay River I-type felsic rocks, which are derived from lower crustal melting generated by mantle heat input and regional exhumation. The intrusion of the final 96–90 Ma Tombstone suite represents the waning of the thermal pulse and the end of exhumation. Plutonism in the Tay River district terminates at 90 Ma. A thermochronological study on these three suites indicates that the intrusion of the two older suites occurred synchronously with the exhumation of the Tay River district. The oldest OMB plutonic rocks were emplaced at 12–15 km depth at 107±2 Ma. These rocks cooled below the 40Ar-39Ar closure temperature in biotite (365–395°C), which corresponds to a depth of 5.5–10.5 km, at ~104±2 Ma. They then cooled below the U-Th/He closure temperature in zircon (195-215°C), which corresponds to a depth of 2.5-4.7 km, before ~99.2 ± 3 Ma. The new definition of suites, based on emplacement age, can be expanded to previously published data from the northern Cordillera to create a new temporal pattern of pluton evolution. The oldest S-type plutons form a core in the regions with the thickest crust and a fertile protolith. Regional I-type plutonism occurs on the peripheries of the Yukon-Tanana terrane. Finally 96–90 Ma plutonism occurs across the whole northern Cordillera. This pattern of plutonism can be explained by delamination of over-thickened lithosphere beneath the Yukon-Tanana terrane. In this model, lithospheric delamination drives regional uplift, as well as creating space for upwelling asthenosphere to transfer heat above the Moho generating lower crustal melts. The development of mineralization at the Keglovic deposit can now be reexamined within the context of this enhanced regional geological understanding. A Re-Os model age of arsenopyrite from within the vein system yields an age of 100±2 Ma indicating that mineralization occurred synchronously with the exhumation, extensive OMB plutonic activity, and the delamination event. Carbon, oxygen, sulphur and lead isotope analyses suggest a purely crustal origin for the mineralizing fluid with no direct magmatic input and that the same fluid formed both the skarn and the subsequent vein mineralization. The superposition of the mineralization styles is therefore due to telescoping of a single hydrothermal system as a result of regional exhumation. Fluid flow and skarn mineralization were controlled by a fault with hydrothermal circulation resulting from a high geothermal gradient derived from crustal thickening, intrusive igneous activity and/or delamination-derived lower crustal heating. The change in mineralization style from the skarn to the veins occurred due to a combination of increased volatile content in the fluids – derived from reactions with the host rock - and decreasing pressure due to regional exhumation. In combination, these two effects led to effervescence within the fluid, which promoted deposition of the veins. In summary, the Keglovic deposit is a product of the regional geological evolution of the Tay River district. Therefore, any future exploration for Keglovic-type deposits will need to consider the broader geological context of the mineralization.

• Subjects / Keywords

  • Yukon
  • Northern Cordillera
  • Economic Geology

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3Q23RD6K

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.